1. Field of the Invention
The present invention relates to a non-sintered nickel electrode such as that which is used in secondary cells containing an alkaline electrolyte, such as, for example, nickel-cadmium, nickel-iron, nickel-hydrogen and nickel-hydridable-metal rechargeable batteries, and to the cell containing it.
2. Description of the Prior Art
Several types of electrode exist, in particular sintered electrodes and non-sintered nickel electrodes, also referred to as impasted or plasticized electrodes. The electrodes most widely used nowadays are of the non-sintered type. Compared with other electrodes, a non-sintered electrode contains a larger amount of active material, its specific capacity is thus increased and its manufacturing cost is lower.
A non-sintered electrode is composed of a support which serves as a current collector, which is coated with a paste containing the active material and a binder, to which is usually added a conductive material. It is usually made by depositing the paste in a porous three-dimensional conductive support such as a felt or a foam made of metal or carbon.
European Patent Application EP-0 726 607 mentions an electrode comprising a porous support coated with a paste. The support is not a critical component, and can be two-dimensional or three-dimensional. The paste contains the active material, a conductive agent, a fluorinated resin and a thickener. The fluorinated resin used as binder can be a mixture of fluorinated resin and of thermoplastic resin. This document does not mention the electrochemical functioning of the electrode.
For cost reasons, the trend is nowadays toward use of a two-dimensional conductive supports.
Japanese Patent Application JP-3 165 469 proposes a nickel electrode comprising a two-dimensional porous conductive support coated with a paste containing nickel hydroxide, a conductive material and a thermoplastic binder, such as a butylene/ethylene/styrene copolymer. In order to ensure binding of the active material to the support, a separator is hot-pressed onto each face of the electrode.
European Patent Application EP-0 750 358 describes a non-sintered nickel electrode whose support is a corrugated metal sheet on which teeth have been formed in order to catch in a layer with microrugosity. A paste comprising carboxymethylcellulose (CMC) and a styrene/butadiene copolymer (SBR) is deposited on this layer.
The known binders used to make an electrode containing a three-dimensional support prove to be unsuitable for a two-dimensional support. In the two previous examples, it was necessary to use a means other than the binder to give the electrode its mechanical strength.
The object of the present invention is to propose a non-sintered nickel electrode containing a two-dimensional, or flat, support, whose mechanical strength and chemical resistance to electrochemical oxidation are enhanced.
The subject of the present invention is a non-sintered nickel electrode containing a two-dimensional conductive support and a paste comprising an electrochemically active material containing nickel hydroxide and a binder which is a mixture of an elastomer and a crystalline polymer, wherein the proportion of the elastomer is in the range 25% to 60% by weight of the binder and the proportion of the crystalline polymer is in the range 40% to 75% by weight of the binder.
The binder is an essential component of the electrode since its role is both mechanical and electrochemical.
The binder has the function of ensuring cohesion of the grains of active material with each other and with the electrode support, before assembly of the rechargeable battery and during its functioning. On the one hand, the binder must be of sufficient chemical stability. Firstly, it must be chemically inert with respect to the components of the cell; next, it must be capable of withstanding electrochemical oxidation under the cycling conditions to which the electrode is subjected. However, certain binders suffer degradation of their adhesive properties during cycling. On the other hand, the binder must be capable of being deformed in order to match the variations in size of the electrode during cycling throughout its life.
The function of the binder is also to maintain the electrical contact between the grains of active material and to promote the ionic exchanges with the electrolyte. On the one hand, the electrochemically active area of an electrode depends on the area wetted by the electrolyte. To promote the wetability of the electrode by the aqueous electrolyte, the binder should have a hydrophilic nature. If the electrode is not sufficiently wetted, the active area is decreased, which leads to an increase in the local current density and a lower charged capacity. On the other hand, the area accessible to the electrolyte depends on the manner in which the grains of active material are coated and bound by the polymer. The polymer film should have discontinuities to allow the electron exchanges.
An elastomer is a polymer which has elastic properties. It is defined as a polymer which has a viscoelastic state at ambient temperature Ta, which means that its glass transition temperature Tg is below ambient temperature Ta. The use of an elastomer as a binder makes it possible to obtain a nickel electrode with suitable mechanical properties. However, when used alone, it forms a film which coats the grains of active material and greatly reduces the electrical conductivity of the electrode.
The elastomer is preferably chosen from a copolymer of styrene, of ethylene, of butylene and of styrene (SEBS), a terpolymer of styrene, of butadiene and of vinylpyridine (SBVR) and a copolymer of styrene and of butadiene (SBR). The copolymer of styrene and of butadiene preferably contains from 25 to 35% by weight of styrene.
In contrast with other elastomers, a crosslinkable elastomer will instead form lumps of polymer distributed on and around the grains of active material. Crosslinking makes it possible to limit the creep of the polymer. Advantageously, the elastomer is a crosslinkable carboxylated copolymer of styrene and of butadiene (carboxylated SBR), i.e. an SBR bearing xe2x80x94COOH groups which allow its crosslinking. A crystalline polymer is characterized by the fact that it has a melting point.
This polymer is solid at ambient temperature. A crystalline polymer does not form a film: when used alone, it does not have sufficient cohesion to keep the active material on the support.
Preferably, the crystalline polymer is chosen from a fluorinated polymer and a polyolefin, such as polyethylene (PE).
If the crystalline polymer is a fluorinated polymer, it is preferably chosen from a fluorinated copolymer of ethylene and of propylene (FEP), polytetrafluoroethylene (PTFE) and polyhexafluoropropylene (PHFP).
From a mechanical point of view, the greater the proportion of elastomer, the better the cohesion of the electrode. The addition of a crystalline polymer has the function of breaking the continuity of the elastomer film, and thus of preserving the electrochemical performance of the electrode. The binder according to the invention should contain at least 25% by weight of elastomer. Below this proportion, the mechanical strength of the electrode will no longer be sufficient, in particular in the case of a coiled electrode.
To ensure the cohesion and electrochemical functioning of the electrode throughout its period of use, the proportion of the crosslinkable elastomer should be in the range 25% to 60% by weight of the binder, and the proportion of the crystalline polymer should be in the range 40% to 70% by weight of the binder.
According to a preferred embodiment, the binder is composed of 40% to 60% by weight of the elastomer and of 40% to 60% by weight of the crystalline polymer.
If a high content of binder is introduced into the paste, the electrical conductivity of the electrode is decreased, which produces a lowering of the energy density of the power source. It is thus essential to minimize the inevitable loss of capacity which results from producing the electrode, by striving to introduce the minimum possible amount of binder required, but a sufficient amount to ensure this cohesion. Below 0.7% by weight of binder, the mechanical strength of the electrode is no longer satisfactory. The proportion of binder according to the invention is preferably in the range 0.7% to 3% by weight of the paste.
An advantage of the present invention is that only a small mass of binder is needed to ensure the cohesion of the electrodes, while at the same time conserving their excellent flexibility, in particular in the case of a coiled electrode.
It is clearly understood that the term xe2x80x9celectrochemically active material containing nickel hydroxidexe2x80x9d used in the present patent application can mean a nickel hydroxide, a hydroxide containing mainly nickel, but also a nickel hydroxide containing at least one syncrystallized hydroxide of an element chosen from zinc, cadmium and magnesium and at least one syncrystallized hydroxide of an element chosen from cobalt, manganese, aluminum, yttrium, calcium, strontium, zirconium and copper. A syncrystallized hydroxide contained in the nickel hydroxide is a hydroxide which forms a solid solution with the nickel hydroxide, i.e. one which occupies, in continuously variable proportion, the atomic sites defined by the crystal lattice of the nickel hydroxide.
Preferably, this hydroxide has a spheroidal shape and has a particle size of in the range 7 xcexcm to 20 xcexcm.
The active material can be coated with a coating based on cobalt oxide or hydroxide optionally containing other elements such as nickel, zinc, aluminum and/or manganese, or alternatively with a porous metal coating, for example of nickel.
Nickel hydroxide is a poorly conductive compound which requires the addition of a conductive material to give good electrical percolation. The paste also comprises a conductive material chosen from conductive particles, conductive fibers and mixtures thereof.
The term xe2x80x9cparticlexe2x80x9d is intended to mean a very small amount of material, the three spatial dimensions of which are of the same order of magnitude, defined by an average dimension D1. The shape of the conductive particles can be spherical, quasi-spherical or completely irregular. Preferably, the conductive particles have an average dimension D1 less than or equal to D/20, D being the average diameter of the grains of the active material, and more preferably D1 is less than or equal to D/100.
The conductive particles are chosen from carbon particles, particles of metal, for example such as nickel, and powder of a transition metal compound such as Co, CoO or Co(OH)2.
The term xe2x80x9cfiberxe2x80x9d is intended to refer to a very small amount of material, one spatial dimension of which is large compared with the other two, defined by an average transverse dimension D2 and an average length L2. Preferably, the conductive fibers have an average transverse dimension D2 less than or equal to D and an average length L2 greater than or equal to 25 times the value of D2, and preferably L2 is greater than or equal to 75 times D2. The conductive fibers are chosen from carbon fibers, metal fibers or fibers coated with metal, for example such as nickel.
According to a preferred embodiment, D1 will be chosen to be less than or equal to 0.1 xcexcm and D2 less than or equal to 2 xcexcm.
Preferably, the proportion of the conductive material in the paste is in the range 3% to 15% by weight of the active material. Above this value, the specific capacity of the electrode decreases on account of the large proportion of conductive material in the electrode.
The paste can also contain at least one compound chosen from zinc compounds, such as ZnO or Zn(OH)2, yttrium compounds such as Y2O3 or Y(OH)3 and calcium compounds such as CaO, Ca(OH)2 or CaF2. This compound is usually added in powder form.
In order to make the electrode easier to produce, the paste also comprises a thickener which is a cellulose compound chosen from carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), hydroxypropylcellulose (HPC) and hydroxyethylcellulose (HEC).
The term xe2x80x9ctwo-dimensional supportxe2x80x9d is intended to refer to a flat support on which a layer of paste is deposited. The adhesive properties of the binder are thus essential for keeping the layer on the support, in particular in the case of coiling of the electrode. The two-dimensional conductive support can be a solid or perforated strip, an expanded metal, a mesh or a fabric. It is, for example, a nickeled steel strip in the range 3xc3x9710xe2x88x923 mm to 10xe2x88x922 mm in thickness, with a mass per unit area of in the range 3 g/dm2 to 6 g/dm2 and an aperture content of in the range 0% to 80%, the diameter of the holes being in the range 0.1 mm to 3 mm. According to one variant, the support can also be a foam with a very low mass per unit area, of 2 g/dm2 to 7 g/dm2, containing the above strip at its center.
An advantage of the present invention is the convenience of manufacture of the electrodes, since most of the polymers can be used in the form of an aqueous emulsion; the manufacture of the electrodes by wet-route impasting followed by drying is facilitated (stable paste with good Theological properties).